Ken-
This is a little different than a rigid trailer, but an interesting project nonetheless.
I made a portable covered frame for my trailer a few years ago.
Ran 2 1/2" PVC Schedule 40 tube down each side of the trailer from front to rear.
Drilled 1/2 inch vertical holes in the tube from front to rear about every 14 inches.
Took 20 1/2" PVC Schedule 40 tubes and inserted 1/2" dowels about 8" long in one end of each of them.
Put the dowels in the hole in the large pipe on one side of the trailer, then bent the pipe into a bow high enough to clear the boat and keep a plastic cover off of it.
Cut the other end of each 1/2" pipe off to the necessary indicated length and inserted another dowel in that end which was then put into the appropriate hole on the opposite side of the trailer.
Repeat for the rest of the bows. Makes a "covered wagon" frame tapered to the shape of the boat that will hold a plastic cover over but not on the boat so as to not rip the cover on the sharp corners of the boat, and to also prevent pooling of water or snow.
Of course, you'll have to stretch a plastic cover over the trailer, boat, and ribs. Secure with bungee cords to tension.
Makes a great and cheap covered housing for storing the boat outdoors and out of the sun, rain, and snow. Not strong enough for on road use, however.
Be sure to number the matching holes and ribs for easy removal and replacement, since the ones up front will be shorter than the ones to the middle and rear in gradually tapered lengths.
The 2 1/2" larger PVC tube is secured to the trailer frame and guide-ons with large U-bolts.
I added a 8" tube down one side of the trailer with removable end plugs in which to store all of the 1/2" ribs during transit.
This works great to store your boat in at a remote location for a few months or so during the summer when you don't want to drag it back and forth to a lake you fish at frequently. I used it at Eagle Lake in California one summer.
I don't use it much any more with a covered marina slip and an RV barn in which to store the boat.
I'll have to get some pictures of the set-up if anyone indicates interest. Will involve bringing the trailer home from friend's where it's stored and assembling it.
Fun, simple, doable project!
Joe.